I have also lost performance in connection with a lot of stuttering overall.
I also realised that every time the reflections start to flicker (Flickering reflections on windshields in cockpit view) my FPS drop to roughly 9 for a short moment. Maybe there is a connection between the flickering and the overall loss of performance.
Edit: As it has been recommended a lot of times, I might add, that changing settings does not resolve the issues.